1. Field of the Invention
The present invention relates in general to a drill, and more particularly to techniques for providing a drill with a long tool life, by reducing a thrust load applied to the drill during a drilling operation for drilling a hole, and also by reducing a generation of friction heat, a wear of the drill and a cutting torque which are caused or increased by frictional contact of margins of the drill and an inner surface of the hole during the drilling operation.
2. Discussion of the Related Art
There is widely known a drill which has cutting lips or edges formed in its axially distal end portion, and chip evacuation flutes formed to extend generally in its axial direction. The drill is used to originate or enlarge a hole in a workpiece, by rotating the drill about its axis and moving the drill and the workpiece relative to each other in the axial direction, so that the workpiece is cut with the cutting edges while chips are allowed to be evacuated from the hole through the flutes. Such a drill has margins which extend along the respective flutes and which have an outside diameter substantially equal to that of the cutting edges. It is common that a cylinder defined by a rotary trajectory of the margins is slightly back-tapered so that the diameter of the margins is gradually reduced as viewed in a direction away from the axially distal end portion toward the axially proximal end portion of the drill (i.e., toward a shank portion of the drill). JP-A-H7-308814 (publication of unexamined Japanese Patent Application laid open in 1995) discloses a drill in which a point thinning or web thinning is made in its chisel edge such that a radially inner portion of each of the cutting edges is ground to provide a secondary cutting edge, while an inclined surface that is contiguous to a rake surface of the secondary cutting edge is formed. This inclined surface is inclined toward the shank portion as the inclined surface extends from the rake surface toward the periphery of the drill in a direction substantially perpendicular to the secondary cutting edge.
FIG. 9A is a perspective view of a distal end portion of such a conventional drill, while FIG. 9B is a cross sectional view taken along line 9B—9B of FIG. 9A. This conventional drill has a pair of cutting lips or edges 100, and first and second flank surfaces 102, 104 which are formed on a rear side of each of the cutting edges 100 as viewed in a rotating direction of the drill. Oil holes 106 are formed throughout the entire axial length of the drill, and open in the respective second flank surfaces 104. The cutting edges 100 are provided by axially distal open ends of respective chip evacuation flutes 108, which are formed in a body of the drill for allowing chips to be evacuated from a hole therethrough during a drilling operation with the drill. A pair of radially inner cutting edges 112 are formed in a radially inner side of the respective chip evacuation flutes 108 by using a thinning grinding wheel 110, as shown in FIG. 9B, such that each of the radially inner cutting edges 112 as a secondary cutting edge is located on a radially inner side of the corresponding cutting edge 100 as a primary cutting edge and is contiguous to the corresponding cutting edge 100. This type of web thinning is commonly called as “Radial Point Thinning” or “R-type Thinning” since the radially inner or secondary cutting edge 112 is formed to have a predetermined radius of curvature. The thinning grinding wheel 110 has an axial end surface and an outer circumferential surface which intersect each other at an angle α1 of about 110°. A rake surface 114 of the secondary cutting edge 112 and a web-thinning bottom surface 116 are formed concurrently with each other by the axial end surface and the outer circumferential surface of the thinning grinding wheel 110, respectively.
An inclined surface 120 is formed by using another grinding wheel 118, so as to extend from the web-thinning bottom surface 116 up to the periphery of the drill. This inclined surface 120, serving to evacuate chips produced by cutting of a workpiece with the secondary cutting edge 112, is inclined toward the shank portion as the inclined surface 120 extends from the bottom surface 116 toward the periphery of the drill in the rightward direction as seen in FIG. 9B, i.e., in a direction substantially perpendicular to the secondary cutting edge 112. In other words, an axial distance between the inclined surface 120 and the proximal end of the drill is gradually reduced as viewed in the direction away from the rake surface 114 toward the periphery of the drill. The inclined surface 120 has a lager clearance angle than the second flank surface 104, and is located on a rear side (as viewed in a rotating direction of the drill) of the second flank surface 104 and is contiguous to the second flank surface 104. The grinding wheel 118 has an axial end surface and an outer circumferential surface which intersect each other at an angle α2 of about 100°. The inclined surface 120 is formed by the outer circumferential surface of the grinding wheel 118.
In the above-described conventional drill, however, since the web-thinning bottom surface 116 and the inclined surface 120 formed by the respective different grinding wheels have respective different shapes, a protrusion or step 122 is inevitably formed along an entirety or part of a boundary between the two surfaces 116, 120. In other words, since the bottom surface 116 has a complicated three-dimensional geometry, it is extremely difficult to smoothly connect the inclined surface 120 to the bottom surface 116 without a step in the entirety of the boundary between the two surfaces 116, 120. The step 122 impedes the evacuation of the chips, making it difficult to reduce a thrust load applied to the drill during a drilling operation, thereby making it impossible to provide a satisfactorily prolonged tool life. It might be possible to reduce the thrust load, by increasing a depth of each chip evacuation flute 108 and reducing a web thickness of the drill. However, a considerable reduction in the web thickness over the entire length of the drill body leads to an undesirable reduction in the rigidity or mechanical strength of the drill, failing to provide a technical advantage favorable for an increase in the tool life after all.
Further, the above-described conventional drill suffers from other problems that a cutting torque is undesirably increased due to a friction generated by contact of margins of the drill with an inner circumferential surface of the hole, thereby deteriorating a cutting performance of the drill, and that a friction heat and a wear of each of the margins are generated and caused by the contact of the margins and the inner circumferential surface of the hole, thereby reducing the tool life, i.e., the number of the holes which can be drilled without regrinding or resharpening the cutting edges of the drill. It might be possible to reduce the cutting torque and the friction heat, by reducing a width of each margin of the drill, namely, by reducing an area of the contact of the margins and the inner circumferential surface of the hole. However, a considerable reduction in the margin width leads to a reduction in the strength of each margin, probably causing fracture at a boundary between the margin and the flank surface of a land (which is located on the rear side of the margin), and the consequent chipping of a leading edge (which is provided by the rear-side one of widthwise opposite edges of the chip evacuation flute).